Japanese Patent Application Laid-open No. 2007-286627 discloses a liquid crystal display device including a direct type backlight unit. In the liquid crystal display device, a plurality of light emitting diodes are used as light sources of the backlight unit. The light emitting diodes are arranged in matrix across an entire region of the backlight unit.
Further, Japanese Patent Application Laid-open No. 2007-305341 also uses a direct type backlight unit in which a plurality of LEDs are arranged in matrix as light sources.
In the liquid crystal display device described in Japanese Patent Application Laid-open No. 2007-286627, the light emitting diodes are arranged across the entire region of the backlight unit, and hence the size of a substrate on which a large number of light emitting diodes are arranged needs to be large enough to cover the entire region of the backlight unit. This increases cost for preparing a large number of light emitting diodes as well as a material cost of the substrate on which the light emitting diodes are to be arranged.
Also in the structure in which the plurality of LEDs are arranged in matrix as exemplified by the backlight unit described in Japanese Patent Application Laid-open No. 2007-305341, the necessary number of LEDs is large to have the same problem.
To address the problem, studies have been made on the structure in which the light emitting diodes are arranged at a part of the backlight unit along a particular direction, that is, the LEDs are arranged only at a vertical or horizontal center portion of the backlight unit, and a reflection sheet is used to expand light of the LEDs in the vertical or horizontal direction. For example, it is conceivable to arrange the light emitting diodes in the vicinity of the lateral center of the backlight unit in a concentrated manner along the long-side direction of the backlight unit, and to reflect or diffuse light beams of the light emitting diodes with use of an appropriate reflection sheet so that the light beams may irradiate an entire image formation region. This structure, however, needs to increase the arrangement density of the LEDs, and hence temperature of the LEDs is liable to be high.
FIG. 9 is a partial enlarged plan view illustrating how a plurality of light emitting diodes 13 are arranged linearly on a light emitting diode substrate 7. Note that, a lens 14 for diffusing a light beam is arranged on a light emitting surface side of each light emitting diode 13. In FIG. 9, parts located behind the lens 14 are illustrated by broken lines. As illustrated in FIG. 9, the light emitting diodes 13 are arranged in series in the longitudinal direction of the liquid crystal display device, that is, in the horizontal direction of FIG. 9.
In this case, an electrode 21 connected to an anode and a cathode of each light emitting diode 13 is spread in plan to have a so-called solid pattern as illustrated in FIG. 9. This is for the purpose of diffusing and dissipating heat from the light emitting diode 13 owing to the electrode 21 having high heat conductivity. Upper limits of the maximum output and the arrangement density of the light emitting diodes 13 are determined based on such heat dissipation ability of the electrode 21. In other words, as the electrode 21 has higher heat dissipation performance, the light emitting diode 13 with a higher output can be used for the light emitting diode substrate 7 of the same area, or the light emitting diodes 13 can be arranged at higher density. Alternatively, when the output of the light emitting diode 13 is the same, the size of the light emitting diode substrate 7 can be reduced more as the electrode 21 has higher heat dissipation performance.
By the way, heat generated from the light emitting diode 13 is transferred to the electrode 21 via the anode and the cathode, but in a light emitting diode 13 commonly used at present, the transfer amount of heat is not equal between the anode and the cathode. In the example illustrated in FIG. 9, no consideration is made on such circumstances and the electrode 21 has the same pattern on the side connected to the anode and on the side connected to the cathode. One of the anode and the cathode having a larger amount of heat generation with higher temperature is referred to as “high temperature side electrode”, and another having a smaller amount of heat generation with relatively lower temperature is referred to as “low temperature side electrode”. In this case, the maximum output and the arrangement density of the light emitting diodes 13 are determined based on heat dissipation performance for the high temperature side electrode subjected to severe thermal conditions. In such a case, heat dissipation performance for the low temperature side electrode has room to improve, and hence the heat dissipation performance has room to further improve. Note that, in most light emitting diodes used at present, the amount of heat generation at the cathode is larger than the amount of heat generation at the anode. In other words, the amount of heat transferred to the electrode 21 is larger on the cathode side and its temperature becomes higher as well.
Alternatively, depending on the size of the liquid crystal display device, it is sometimes necessary to arrange the light emitting diodes in two or more rows, because a sufficient amount of light for illuminating the entire image formation region cannot be obtained by simply arranging the light emitting diodes linearly in one row.
FIG. 18 is a partial enlarged plan view illustrating how the plurality of light emitting diodes 13 are arranged linearly in two rows on the light emitting diode substrate 7. Note that, the lens 14 for diffusing a light beam is arranged on the light emitting surface side of each light emitting diode 13. In FIG. 18, parts located behind the lens 14 are illustrated by broken lines. As illustrated in FIG. 18, the light emitting diodes 13 are arranged in two rows in series in the longitudinal direction of the liquid crystal display device, that is, in the horizontal direction of FIG. 18, and in parallel in the lateral direction of the liquid crystal display device, that is, in the vertical direction of FIG. 18. The light emitting diodes 13 are arranged in the respective rows at staggered positions. This is for the purpose of obtaining as uniform illumination as possible in the longitudinal direction. In such a case, the arrangement density of the light emitting diodes 13 becomes higher, and the temperature of the light emitting diode 13 is liable to be higher.
Further, in the case where the plurality of LEDs are arranged in three or four rows at the vertical or horizontal center portion, the temperature of the LEDs arranged in the middle row(s) sandwiched by the two rows on both sides is liable to be high.
The present application has been made in view of the above-mentioned circumstances, and it is an object thereof to efficiently dissipate heat generated from a light emitting diode in a liquid crystal display device including a backlight unit for irradiating an entire image formation region with light of light emitting diodes arranged in a concentrated manner.
It is another object of the present application to provide a liquid crystal display device capable of improving heat dissipation performance for LEDs and a television receiver including the liquid crystal display device.